1. Field of the Invention
The present invention relates to array antenna systems having the capability to transmit and receive, for example, a radar signal, as well as to detect the direction of incident waves.
2. Description of the Prior Art
Electronically beam scanning array antenna systems are well known in this technical field. In such an array antenna system, antenna elements are arranged along a line or on a surface of a predetermined shape and are connected to the respective phase shifters. The direction of the beam formed by the array antenna system can be determined by controlling the amount of phase shift set to the respective phase shifters. On the other hand, the directivity of the array antenna system can, as is well known, be determined by the aperture distribution of the current. For example, the amplitude of the current excited in each antenna element is determined to produce, for example, the Taylor distribution or the Tchebycheff distribution on the aperture.
T. Numazaki et al. "An Improved Method for Density Tapering of Planar Array Antennas" IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, Vol. AP-35, No. 9, September., 1987, p. 1066-1070 shows a phased array antenna system which can produce desired aperture distribution by a thinning method, i.e., by connecting some selected antenna elements to terminating devices such as resistors instead of to a transmitter or a receiver, thereby producing a predetermined density distribution of antenna elements required for establishing an aperture distribution equivalent to the Taylor distribution, the Tchebycheff distribution or the like. Accordingly, array antenna systems having different directivities can be obtained and used for various functions, such as for radars in searching or tracking airplanes.
However, such electronically scanning array antenna systems have the problem of not being able to accurately locate a target.